I know how to saturate in post-processing. But there are times of the day and the year when natural outdoor light is extremely saturated. Towards sunset in spring in southern England, for example. What is the physical quality that makes this light "saturated"? Is it a change in colour temperature? Or is there something else which affects perceived saturation?

2 Answers
2

I think chills42 is correct, a few factors compound to create vivid colors when the sun is low on the horizon. This affects the actual scene, i.e. the actual colors we see.

But there is another dimension to the situation: what the photos we take look like. The dynamic range of the scene during the golden hours is usually lower, allowing the camera to capture colors more easily. This happens to me all the time; a scene in bright noon sunlight looks completely vivid and saturated (imagine a field of flowers on a sunny day with a blue sky), but there is no way to capture it on a camera with a single exposure. I can underexpose the sky to capture the vivid blues, but then the foreground will be underexposed. Or I can expose the foreground correctly and the sky will overexpose and look white. I think this tricks some people (when they are viewing their photos) into thinking the scene had less color than it did.

Thank you for this @rm999 - it seems to wrap up all the answers into a ball of thoughtfulness. But aren't vividness and saturation slightly different qualities? I have a picture showing exactly your example but I see now it was taken in the evening, which explains why it is so saturated.
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Andrew BrownFeb 27 '11 at 12:23

Typically, lighting is softer (more
diffuse) and warmer in hue, and
shadows are longer. When the Sun is
near the horizon, sunlight travels
through more of the atmosphere,
reducing the intensity of the direct
light, so that more of the
illumination comes from indirect light
from the sky (Thomas 1973, 9–13),
reducing the lighting ratio. More blue
light is scattered, so that light from
the Sun appears more reddish. In
addition, the Sun's small angle with
the horizon produces longer shadows.

These times are often referred to as the "golden hours", and there is an online calculator that can tell you the times that the light will be best in your area.

Another big component is the huge reduction in UV light caused by the longer pathway through the atmosphere. Sensors aren't bothered by UV in the same way film is, but a lot of things in nature fluoresce somewhat under UV, making them not only reflectors but emitters of light, and that can significantly reduce the saturation of reflected colours.
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user2719Feb 26 '11 at 20:43